Alkenyl-substituted aromatic compounds are important starting materials for the production of resins, plastics, rubbers, solvents, chemical intermediates, and the like.
Processes for the production of alkenyl-substituted aromatic compounds often are characterized by low conversion rates which necessitate the recycle of large quantities of unconverted charge. Many of the known processes require the presence of a large volume of steam or other gaseous diluent which is a cost disadvantage. In some processes the conversion efficiency to alkenyl-substituted aromatic product is diminished because of the formation of a relatively large proportion of carbon oxides and other byproducts.
In one well-known commercial process, C.sub.2 -C.sub.3 alkylaromatic hydrocarbons (e.g., ethylbenzene, ethyltoluene and isopropylbenzene) are converted to the corresponding styrene derivatives by passage of the alkylaromatic hydrocarbon feed and steam over a Fe.sub.2 O.sub.3 catalyst. The conversion per pass is in the 35-40% range, and comparatively high temperatures are needed for the oxidative dehydrogenation reaction.
Illustrative of other oxidative dehydrogenation processes, U.S. Pat. No. 3,299,155 describes a process for the production of alkenylbenzenes which involves contacting a mixture of an ethyl (or isopropyl) substituted benzene compound and sulfur dioxide in vapor phase with a metal phosphate catalyst such as calcium phosphate.
U.S. Pat. No. 3,409,696 describes a process which involves contacting an admixture of C.sub.2 -C.sub.4 alkylaromatic hydrocarbon and steam at a temperature of 500.degree.-650.degree. C. with a catalyst containing 20-60 weight percent of a bismuth compound (e.g., bismuth oxide) on a calcium phosphate support of which at least 90% of the total pore volume is contributed by pores having a diameter of 1000-6000 A.
U.S. Pat. No. 3,733,327 describes an oxydehydrogenation process for converting a C.sub.2 -C.sub.6 alkylaromatic compound to the corresponding C.sub.2 -C.sub.6 alkenylaromatic compound which comprises contacting an admixture of starting material and oxygen at 400.degree.-650.degree. C. with a cerium phosphate or cerium-zirconium phosphate catalyst.
U.S. Pat. No. 3,957,897 describes a process for oxydehydrogenation of C.sub.2 -C.sub.6 alkylaromatic compounds which involves the use of oxygen, a reaction zone temperature of 450.degree.-650.degree. C., a space velocity of 55-2500, and a catalyst which is at least one of calcium, magnesium and strontium pyrophosphate.
More recently, there has been increasing concern with respect to the potentially harmful environmental effects associated with the manufacture of synthetic resin products. In the molding of large shaped articles, for example, volatile components of a polymerizable monomeric formulation sometimes tend to evaporate from freshly coated mold surfaces which are exposed.
Various means have been contemplated for reducing the level of fugitive vapors in a synthetic resin manufacturing plant. One method involves the replacement of volatile monomers of a formulation with monomers which have a lower vapor pressure. Thus, it is advantageous to substitute an alkenylaromatic compound such as tertiary-butylstyrene for styrene in a polymerizable formulation which contains the volatile styrene as a comonomer.
As a further consideration, it has been found that tertiary-butylstyrene is desirable as a comonomer in the preparation of copolymers or as a curing agent for fiber-reinforced plastics because it improves the moldability of polymerizable formulations and it lessens the mold shrinkage of molded plastic articles.
The advantages of tertiary-butylstyrene as a comonomer in resin systems has stimulated interest in improved processes for synthesizing this type of higher molecular weight alkenylaromatic compound.
U.S. Pat. No. 3,932,549 describes a process for preparing tertiary-butylstyrene which comprises reacting tertiary-butylbenzene with ethylene and oxygen at 50.degree.-300.degree. C. in the presence of a catalyst prepared by treating metallic palladium or a fatty acid salt thereof with pyridine.
Other known processes for producing tertiary-butylstyrene involve oxydehydrogenation of tertiary-butylethylbenzene. The type of patent processes described hereinabove for oxydehydrogenation of C.sub.2 -C.sub.6 alkylaromatic compounds are generally applicable for conversion of tertiary-butylethylbenzene to tertiary-butylstyrene.
However, the chemical reactivity of tertiary-butylethylbenzene under oxydehydrogenation conditions is more complex than that of simpler chemical structures such as ethylbenzene or ethyltoluene. The tertiary-butyl substituent of tertiary-butylethylbenzene under oxydehydrogenation conditions is susceptible to cracking so as to yield methane and a residual isopropenyl substituent on the benzene nucleus. Consequently, one of the ultimate byproducts of tertiary-butylethylbenzene oxydehydrogenation is a dialkenylbenzene derivative such as isopropenylstyrene.
Because of the presence of two or more polymerizable alkenyl groups, a compound such as isopropenylstyrene tends to undergo crosslinking activity and form insoluble byproducts during the high temperature cycles of starting material conversion and product recovery in an oxydehydrogenation process. Heat exchangers and distillation columns can be rendered inoperative by the deposition of high molecular weight polymeric residues.
Further, the presence of an isopropenylstyrene type of contaminant, particularly a variable quantity of such material, in purified tertiary-butylstyrene can complicate or even prohibit the application of the contaminated tertiary-butylstyrene product as a comonomer in polymerizable formulations.
Accordingly, it is an object of the invention to provide a process for oxydehydrogenation of C.sub.2 -C.sub.6 alkyl-substituted aromatic compounds to the corresponding alkenyl-substituted aromatic derivatives.
It is another object of this invention to provide a process for converting tertiary-butylethylbenzene to tertiary-butylstyrene under moderate conditions with a high level of starting material conversion and product selectivity.
It is another object of this invention to provide a process for converting tertiary-butylethylbenzene to tertiary-butylstyrene with little or no production of dialkenylbenzene byproducts.
It is a further object of this invention to provide a novel catalyst adapted for oxydehydrogenation processes.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.